1. Field of the Invention
The present invention relates to an LED control circuit for controlling a plurality of LEDs with different emission colors, and in particular to controlling the brightness of an LED.
2. Description of the Related Art
By using a combination of red (R), green (G), and blue (B) LEDs, i.e. three primary color LEDs, and adjusting the brightness of the LEDs, various emission colors can be produced. Such three color LEDs are incorporated in, for example, cellular and PHS mobile phones. In a cellular phone, for example, upon receipt of an incoming call or a text massage, a corresponding color light is emitted to notify a user of the receipt. As a method of realizing multicolor multi-gradation display using three color LEDs, there has been known a PWM (Pulse Width Modulation) method in which gradation of brightness of LEDs is controlled by adjusting each duty factor of pulsed voltages applied to the LEDs.
FIG. 13 shows an example of a conventional LED control circuit 100 for modulating the brightness of three color LEDs 1r, 1g, and 1b according to the PWM method. In FIG. 13, the LED control circuit 100 is connected at its input terminal to a microcomputer (micon) 200 and connected at its output terminals to the three color LEDs 1r, 1g, and 1b. 
To cause the three color LEDs 1r, 1g, and 1b to enter into their respective desired lighting states, the micon 200 generates a serial data signal SDATA and supplies the serial data signal SDATA to the LED control circuit 100.
In the LED control circuit 100, a serial I/F 110 specifies various setting values in registers 121 to 127 based on the serial data signal SDATA supplied from the micon 200. Specifically, an ON-OFF setting value for turning the LEDs 1r, 1g, and 1b ON or OFF is set in the register 121, and turn-on position setting values for the LEDs 1r, 1g, and 1b are set in the registers 122, 124, and 126, respectively. Turn-off position setting values for the LEDs 1r, 1g, and 1b are set in the registers 123, 125, and 127. The turn-on position setting values define a start position of the ON period in one PWM cycle (for example, 128 of the clock count), while the turn-off position setting values define a start position of OFF period in one PWM cycle. Therefore, the duty factor of the pulsed voltage is determined by the turn-on position setting values and the turn-off position setting values.
A PWM driving circuit 141 generates a PWM signal having a desired duty factor based on the turn-on and turn-off position setting values established in the registers 122 and 123 using a clock signal for PWM generated by a counter 130. When the ON-OFF setting value for the LED 1r specified in the register 121 is “ON”, the generated PWM signal is applied via an amplifier 151 to one terminal of the LED 1r, thereby causing the LED 1r to emit light at a brightness level in accordance with the duty factor of the PWM signal.
Similarly, a PWM driving circuit 142 generates a PWM signal based on the setting values in the registers 124 and 125. Then, when the ON-OFF setting value for the LED 1g specified in the register 121 is “ON”, the generated PWM signal is applied via an amplifier 152 to one terminal of the LED 1g. Further, a PWM driving circuit 143 generates a PWM signal based on the setting values in the registers 126 and 127. When the ON-OFF setting value for the LED 1b specified in the register 121 is “ON”, the generated PWM signal is applied via an amplifier 153 to one terminal of the LED 1b. 
The LEDs 1r, 1g, and 1b are connected at their anodes to a power source Vdd and connected at their cathodes to the amplifiers 151, 152, and 153, and configured to emit light when currents are drawn into the amplifiers 151, 152, and 153.
In the LED control circuit 100 shown in FIG. 13, by individually changing the turn-on and turn-off position setting values for each of the LEDs 1r, 1g, and 1b, the LEDs 1r, 1g, and 1b can differ in brightness level from each other, which enables provision of various emission colors.
In recent years, there has been a growing demand for fade-in and fade-out functions in which the brightness level gradually increases when an LED is turned on and gradually decreases when the LED is turned off. These functions are implemented in the LED control circuit 100 shown in FIG. 13 by gradually extending the ON period for PWM to enable a fade-in, and gradually reducing the OFF period for PWM to enable a fade-out. Specifically, in a state wherein the turn-on position setting value in the register 122 is set at “0”, the turn-off position setting value in the register 123 is increased from “0” to “50” by one, to thereby enable gradual increase in brightness of the LED 1r from 0/128th gradation level (turned-off state) to 50/128th gradation level.
FIG. 14 shows a fade-in and a fade-out achieved in a conventional manner. In FIG. 14, the abscissa represents time and the ordinate represents a duty factor of a PWM signal. In addition, a brightness setting value for the LED 1r (a duty factor in a normal lighting state) is set to “80/128”, while the brightness setting value for the LED 1g is set to “40/128”. On the other hand, the LED 1b is set to the “OFF” state.
With the above-described settings, when fade-ins, for example, of the LED 1r and the LED 1g are simultaneously started, as shown in FIG. 14, the LED 1g reaches the established brightness setting value ahead of the LED 1r, and thereafter brightness of only the LED 1r changes alone. Further, when fade-outs are simultaneously started, for example, the LED 1g reaches the turned off state (a condition where the duty factor is zero) ahead of the LED 1r, which results in a situation that only the LED 1r thereafter remains turned on.
Accordingly, in the conventional manner, when a plurality of LEDs have different brightness setting values, the brightness ratio of the plurality of LEDs could be diverted greatly from a desired brightness ratio in the course of fading in or fading out. In other words, it is not possible to perform fade-in and fade-out operation of desired halftone color light.